The fabrication of integrated circuits on semiconductor wafers typically requires a large degree of precision. With the state of the art feature sizes approaching 45 nanometers and below, a slight misalignment can result in a useless wafer. Since integrated circuits are typically made by fabricating multiple, successive layers, a misalignment of a single layer can result in an inoperable integrated circuit.
Misalignment can arise from several sources, such as an improperly aligned tool, an improperly aligned wafer, and so forth. An improperly aligned tool may be a tool that is used in the fabrication process, such as a holder for a semiconductor wafer or an optical system for a light lithography machine, which can be out of alignment based upon some reference. The reference may be another tool used in the fabrication process or a reference machine (a golden machine). An improperly aligned wafer may be a wafer that has not been inserted properly into a holder or a fabrication machine. The misalignment of the tool or wafer can result in errors such as portions of the integrated circuit being formed in a position that is different from intended, a portion of the integrated circuit being formed properly while another portion of the same integrated circuit is improperly formed, and so forth.
Alignment marks can be used by tools, such as exposure tools, to optically align a tool or a wafer. For example, to align a tool, a reference tool that holds a golden wafer can be used to provide alignment information for a tool being aligned. To align a wafer, alignment marks on the wafer can be used to provide alignment information.
One disadvantage of the prior art is that the alignment marks proposed in the past can be used to detect mechanical shift error and lens magnification errors. However, lens aberrations can also result in significant alignment errors and the prior art overlay targets do not adequately capture lens aberration errors.
A second disadvantage of the prior art is that the alignment marks proposed in the past do not take advantage of using polarized light.